The present invention relates to a feed roll for transporting a material web and measuring a tension of a continually movable material web.
Feed rolls of the type under discussion are utilized in roller rotary printing machines in which a paper tape must be moved in register, and automatic control, adjusting, examining and testing devices are required for this purpose in connection with web transporting devices.
For measurements of the web tension and adjustments of this tension in contemporary roller rotary printing machines are required direct and reliable measuring devices. These requirements are generally fulfilled by known devices in which web tension is determined with the aid of force value-receivers positioned at two sides of the web-guiding roller supported in the bearings. Such force value-receivers can be formed with semi-conductive strain gauge strips, which have been disclosed, for example in DE-PS No. 2,356,009.
The measurement of the tension of the material moved between the printing mechanisms or between the last printing mechanism and the cooling roller aggregate is not possible with conventional devices in which the web-guiding roller is in contact with the outer surface of the movable web because of supplying of a fresh printing ink. Measurement of the web tension as seen in the direction of running of the web, before or behind each processing station, however ensures a loop-free web transportation and therefore registered movement thereof.
Pneumatically-operating devices for measuring the web tension have been also known. In the device disclosed in DE-PS No. 1,129,965, air stream supplied from the nozzles of the container, arranged above or below the movable paper web, and directed against the web causes the deflection of the web in dependence upon the web tension. In order to avoid undesired reactions from the deflection of the web due to change in the distance between the deflected web and the stationarily-positioned nozzles it is also required that the nozzles be reset to change intensity and direction components of the air stream. This has required an automatic control system for a position control.
Further known is a pneumatic device for measuring web tension, in which device an air stream supplied from the nozzles is throttled by the web and a pressure change in dependence upon the web tension is measured. Such a device has been disclosed by R. Ruder, R. Stott, in "Papier und Druck", 30, 1981, 2, "Teil Druck und Verarbeitung", pages 17 to 21.
A paper web is guided, for example over a stationary curved table. From the container positioned below the table air is supplied from a pressure air source under constant pressure. Air outlet openings open in the table over the entire width of the web. These openings are covered by the web. Depending upon the web tension air escapes, and a respective pressure in the container is adjusted.
A small distance between the web and the table does not preclude lubricating with a fresh printing ink, particularly with web fluttering.
A common disadvantage of all pneumatic devices resides in that providing a supply of purified pressure air of constant pressure is relatively expensive.
The lubricating of the fresh print has little influence on the quality of the printing when the length of winding off portion of the roll which is in contact with the upper surface of the movable web is the same or many times greater than the length of the winding off portion of the printing plate cylinder. If the roll positioned in contact with the web is driven not by friction, as is the case with the web-guiding roll, but is driven by the machine and therefore is mechanically connected with the printing plate cylinder the ink lubrication influence is insignificant. It has been therefore recognized that the feed rolls must be arranged behind the last printing mechanism as seen in the direction of course of printing.
An adjuster of web tension has been disclosed in DE-PS No. 1,086,246. In this adjuster, a pair of feed rolls coupled to the machine drive via a pulling drive or transmission are provided. A swinging frame is arranged below this pair of feed rolls, which frame is biased by a counter spring, and a transmission ratio of the drive transmission would be controlled by an oscillating or swinging movement of that frame, which movement depends upon the web tension. The adjustment of this arrangement takes place in a static condition so that web-pulling forces, forces of the pulling means and a restoring force are in equilibrium, and thereby no torque occurs with reference to the swinging frame. A change in a web tension causes a proportional deflection of the swinging frame. This equilibrium condition permits to use the feed roll which transports the web without the counter roll with friction and/or pressing and must be driven by the tooth gear drive or the drive including a pulling means, such as a belt drive or chain drive.